Generally, it is well known that the surface of a steel strip can be plated with zinc or tin by subjecting the strip to an electrolytic treatment. In such electrolytic treatment, a vertical electrolytic apparatus is usually used. In this apparatus, a steel strip is caused to pass through an electrolytic vessel filled with an electrolyte via rubber rolls submerged in the electrolyte and conductor rolls positioned above the surface of the electrolyte. During the passage through the electrolyte, the strip is electroplated by applying a voltage between the strip as a cathode and an electrode plate as an anode which is suspended in the electrolyte in such a manner that its surface faces a surface of the strip.
However, the conventional electrolytic apparatus is designed on the basis of the concept that the strip is caused to be immersed in the electrolyte. Accordingly, it has the following disadvantages from an operational point of view:
(1) In the case where the apparatus is stopped due to any trouble and the electrolyte must be removed from the vessel, the removal of the electrolyte requires a long period of time, which results in a considerable delay before resumption of the operation, because the vessel is filled with a large amount of the electrolyte. PA1 (2) The strip receives an electric current from the conductor roll positioned above the surface of the electrolyte. In this case, because the roll submerged in the electrolyte should be a non-conductive roll, i.e. an insulating roll such as a rubber roll, a current is supplied to the strip from one direction only. Therefore, the strip extending between the adjacent two conductor rolls around the insulating roll exhibits a high electrical resistance which causes a large consumption of electric power. This is undesirable from the viewpoint of achieving a reduction in energy consumption. Also, because a current is supplied to the strip passing between the electrode plates from one direction only, it is necessary to slope the electrode plates with respect to the strip in order to obtain a uniform distribution of current in the electrolyte involved. PA1 (3) In the immersion electrolysis, while a current is not be supplied, a reverse potential due to a difference in standard electrode potential is created between the insoluble electrode and the material to be plated and between the insoluble electrode and the plating metal. As a result, a potential inversion occurs and the anode acts as a cathode, while the cathode acts as an anode. Accordingly, even in the case where an insoluble electrode material is used for the electrode plate, the useful life of the resultant electrode plate is not significantly long. PA1 (4) The path of movement of the strip may fluctuate due to vibration or twist occurring in the strip between the upper roll and the lower roll as well as the non-uniform shape and C-shaped warp of the strip itself in the transverse direction thereof. Therefore, it is impossible to dispose the electrode plates so as to be very close to the strip. In the conventional electrolytic apparatus, the distance between the surface of the electrode plate and the surface of the strip should be in a range of about 30 to 60 mm. Such a large distance results in the use of a high voltage for electrolysis. This is disadvantageous from the viewpoint of a reduction in energy consumption. Also, it is impossible to carry out a high current density electrolysis. PA1 (5) The portion of the electrolyte contained in the space between the electrode plates is not satisfactorily circulated to the other portion of the electrolyte. As a result, the efficiency of electrolysis deteriorates. Also, when the current density is increased, the quality of the resultant plated layer inevitably becomes inferior. PA1 a vessel for defining an electrolytic treatment space for a metal strip; PA1 a plurality of conductor rolls arranged along a path of movement of said metal strip extending through said treatment space; PA1 at least one pair of electrode pads, each pair of electrode pads being located between two said conductor rolls and being spaced from and facing each other with said movement of path of said steel strip, therebetween and each pad being provided with at least one slit through which an electrolyte is ejected toward the surface of said metal strip under conditions adequate for creating a static pressure of said ejected electrolyte sufficiently high for holding said metal strip in its path of movement in the gap between said electrode pad and said metal strip; PA1 means for supplying said electrolyte to each electrode pad, and; PA1 means for applying a voltage between at least one of said conductor rolls and said electrode pads.
Japanese Patent Application Publication No. 52--23985(1977) discloses another type of electrolytic apparatus. In accordance with this apparatus, the plating is carried out at the place where the strip faces a direction-converting roll immersed in the electrolyte. This apparatus is characterized by the fact that the strip is guided out of contact with the roll under the action of a fluid cushion which is provided by the electrolyte injected through holes in the surface of the roll and at the same time, the roll is caused to act as an anode.
However, in this electrolytic apparatus, the plating is also carried out while keeping the strip immersed in the electrolyte. Accordingly, this apparatus gives rise to the same problems as those described for the above described conventional apparatus. Moreover, because only one surface of the strip which faces the surface of the immersed roll is plated in this electrolytic apparatus, when both surfaces of the strip are to be plated, the plating must be carried out after reversing the plated surface of the strip in the second of two apparatuses as described above. For this reason, the electrolytic apparatus inevitably becomes large in size.
In addition, in this electrolytic apparatus, the strip is caused to travel out of contact with the roll by ejecting the electrolyte onto only one surface of the strip through the holes provided in the surface of the roll, thereby maintaining a certain distance between the surface of the strip and the surface of the roll. While the strip is caused to travel along the roll, the distance between the surface of the strip and the surface of the roll always fluctuates depending on the change in the tension of the strip. As a result, it is difficult to obatin a uniform deposition of the plating metal on the strip.